The present invention relates to MTI radar systems and more particularly to new and improved techniques for rejecting multiple clutter types in an MTI radar.
Generally, MTI radar systems distinguish between targets and clutter on the basis of the doppler frequency shift imparted by a moving object. In designing conventional MTI radar systems it was usually assumed that clutter was stationary thereby providing a zero doppler shift. Moving targets could then be distinguished by placing a filter notch at the point where the radar return had zero doppler to cancel any echo from the clutter. While such systems worked well when the clutter was in fact stationary, problems arose when moving clutter existed in the viewing area. More particularly, such factors as radar platform movement and environmental effects (e.g. wind or rain) caused the clutter to become non-stationary and imparted to doppler shift to the radar returns which prevented the conventional MTI systems from effectively cancelling the clutter.
In order to compensate for the incomplete cancellation initiated by the clutter movement, various systems have been proposed which attempt to distinguish a clutter doppler from a target doppler. One such system known as the Time-Average-Clutter Coherent Airborne Radar (TACCAR), as described in the "Radar Handbook" by M. I. Skolnik, McGraw Hill, 1970 chapters 17 and 18, uses a feedback loop to compensate for clutter movement. In this particular system a filter notch is provided at a velocity corresponding to the mean-clutter-doppler by an analog loop which senses any change in the mean-clutter-doppler and shifts the doppler frequency to maintain the filter notch for clutter cancellation. In this manner, the movement imparted to clutter due to external conditions could be continuously compensated to prevent interference with moving target detection.
While the above TACCAR technique has been relatively successful in cancelling moving clutter, many present day radars are implemented digitally and are not compatible with the analog TACCAR technique. In addition, the TACCAR technique is inherently limited to cancelling only one particular clutter type since the IF mixer can only provide a filter notch at one frequency at a time. Since radar returns may contain a variety of clutter types simultaneously, each hving different mean doppler frequencies, it can thus be seen that the TACCAR technique is severly restricted in multiple clutter environments. In long range surveillance radar systems, for example, a major problem is introduced by the simultaneous occurrence of land and weather clutter (or chaff) in the radar returns. These two types of clutter each have an entirely different mean-clutter-doppler frequency. The land clutter usually has a strong and narrow doppler spectrum at zero doppler, and the weather clutter a weaker but wider doppler spectrum at a high mean-clutter-doppler frequency. To effectively cancel the clutter, therefore, filter notches must be placed at each mean-clutter-doppler frequency, a requirement that cannot be implemented with the conventional TACCAR systems.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide a new technique for allowing improved clutter cancellation in digital radars.